Sword of Spirit
Legend
A Monster Slayer will be wanting to use Slayers eye on round 1 (which chews up his bonus action). Now on round 1 he can use Favored Foe AND use his Slayers eye (dropping FF for HM if he wants after Round 1). Ditto with Beastmasters, who likely want to command the Beast to do something on round 1 other than simply Dodge.
That makes sense. Could be useful for those subclasses. I kind of wish they'd include more sidebars explaining intention like they do in some of the UAs. Maybe they will in Tashas. My immediate thought isn't "how can I use this with sub-classes from XGtE?" but "how does this work with the PHB?" It is coming out in the same book that lets Beast Masters use their bonus action instead of their action to command select companions, so maybe that's the intentional tie in.